Automatic signal control system



April 9, 1957 D. E. suNsTElN ET AL 2,788,450

AUTOMATIC SIGNAL CONTROL SYSTEM 3 Sheets-Sheet 1 Filed June 4, 1947 .arllafin 3 Sheets-Sheet 2 D- E. SUNSTEIN ETAL INVENTORS AUTOMATIC SIGNAL CONTROL. SYSTEM April 9, 1957 Filed June 4, 1947 April 9, 1957 D. E. suNsTElN ET AL 2,788,450

AUTOMATIC SIGNAL CONTROL SYSTEM Filed Jne 4, 1947 3 Sheets-Sheet 3 United 2,7ss,4se

AUTOMATIC SIGNAL coNTRoL SYSTEM David E. Sunstein, Cynwyd, and Allen C. Munster, Philadelphia, Pa., assignors to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application .lune 4, 1947, Serial No. '752,370

11 Claims. (Cl. Z50-36) The invention herein described and claimed relates to automatic control systems and, in particular, to lthe class of such systems in which an input signal is modified to yield an output signal which is controlledly related to said input signal, said relationship being determined in response to said output signal, Control systems of this category are applicable generally to the control of mechanical, electrical, thermal and other processes and are frequently referred to as servo systems. Depending upon the kind of system to which the control is applied, the input signal may be, for example, a mechanical motion, an electrical signal or a thermal quantity.

For purposes of this specification it will be convenient to describe and discuss the invention as applied to electrical systems, although it will be apparent that it is equally applicable to mechanical, thermal and other systems.

A typical electrical application ci systems of this sort is in automatic frequency control (AFC) and automatic gain control (AGC) systems such as are widely used in receivers of radio and radar signals. in such receivers, automatic frequency control systems are customarily used to control the frequency of a local oscillator, the output from which is hcterodyned with the received signal to yield an intermediate frequency signal whose carrier frequency must be maintained within predetermined limits corresponding to the limits of the band in which an intermediate frequency amplifier capable of functioning eciently. Likewise, in such receivers, automatic gain control systems are employed to effect substantially an equalization of the strength of received signals of varying intensity.

A wide variety of such systems has been developed, each possessing peculiar characteristics which adapt it for certain specific applications. Each, however, appears to incorporate means of one kind or another for per- (forming two basic functions-namely: l) measuring the departure from a predetermined standard of some particular characteristic of the signal to be controlled, and (2) electing, in response to the magnitude of such measured departure, a diminution thereof. Apparently, also, those systems which have been developed in the past characterized in that the lforce tending to restore the particular characteristic of the controlled signal to standard decreases in magnitude as the value of the predetermined characteristic approaches the standard value. Although this characteristic is in certain respects desirable (i. e. the tendency to overshoot is thereby re ucsd), it also gives -rise -to the disadvantage that the restoration of the controlled signal to standard occurs more slowly than is permissible for certain applications.

For example, this characteristic represents a distinct disadvantage in an l tomatic frequency control arrangement used in the r ver circuits or" a radar system 'to control the frequency of the local ose ator so that the intermediate carrier frequency, resulting from heterodyning -otf the local oscillator signal Awith -the received 2,788,450 Patented Apr. 9, 1957 signal, will always lie within the desired intermediate frequency band. In a radar system of the usual type, in which energy is transmitted in the form of pulses of relatively short duration and relatively widely spaced in time, it is generally feasible for the AFC system to operate only during intervals coinciding in time with the transmitted pulses, and in response to energy contained in said pulses. in general, reliable control cannot be etected in response to the reflections of such transmitted pulses from target objects. This being the case, if the frequency variation of the transmitted signal is not too rapid, a circuit having a long time constant compared to the pulse repetition period may be used to maintain control during the intervals between pulses. This, however, will make the system slow acting and unsuitable for use where the transmitted frequency is susceptible of rapid variation.

Means have been devised in an attempt to correct this difiiculty. For example, in radars operating at extremely high transmitted frequencies (e. g. of the order of 10,000 megacycics), it has been customary to provide means for sweeping the local oscillator freq ency back and forth throughout a range. The control signal is then applied directly to control the sweeping in a manner to maintain the oscillator frequency within prescribed limits. However, even this method is not adapted to provide the precision of control which is required under certain circumstances. This might be the case, for example, in a socalled single-shot object motion indicating radar system of the sort described in copending application of David E. Sunstein, Serial No. 694,297, filed August 3l, 1946, now Patent No. 2,739,307, dated March 20, 1956. In a system of tie sort there described, indications of object motion are produced in response to shifts in the frequency of the energy contained in individual transmitted pulses when they are reflected from moving objects. The effectiveness of such a system could be greatly enhanced by providing means capable of precisely adjusting the local oscillator frequency following each transmitted radar puise. There are, of course, many other instances in which such precision of control is advantageous or necessary, and this is true of signal characteristics other than frequency.

Accordingly the principal objects of the invention are:

(l) To provide improved automatic control systems and methods which are capable of yielding extremely high sensitivity of control;

(2) To provide .improved means for automatically controlling the relationship between input signals to a system and output signals from said system, whereby said relationship is successively redetermined in response to the values of output signals from said system during timespaced intervals;

(3) To provide en improvedautomatic control system in which the relationship between input signals to said system and output signals from said system is continuously controlled in response to the values of said output signals during time-spaced intervals of limited duration;

(4) To provide improved means for automatically controlling the relationship between input signals to a system and output signals from said system whereby said relationship is successively redetermined following recurrent time-spaced intervals in response to the values of output signals from said system during said intervals;

(5) To provide an automatic control system of the class in which an input signal is modilied to yield an `output signal which is controlledly related to said. input signal, said relationship being determined in response to saidroutput signal and in such manner kthat a new relationshipbetween inputtand output signals is substantially established before any change in vthe'output signal owing jto said new relationship is permitted to effect a further alterationrin said relationship;

(6) To provide an automatic control system of the class in which an input signal is modified to yield an output signal which is controlledly related to said input signal, said relationship at any given time being determined in response to the value of the output signal at some preceding time and being substantially independent of the change in said relationship between said times.

The general method of achieving these obi'ectives according to the invention will most readily be understood by considering the application of the invention to a specie system-an automatic frequency control system employing a Vconventional local oscillator and mixenwhereby the relationship between the frequencies of input signals to said mixer and output signals from said mixer is controlled, by appropriately varying the frequency of the local oscillator signal supplied to the mixer, in a manner to maintain the frequency of the output signal from the mixer substantially constant. As in conventional AFC systems, a discriminator is provided to measure the deviation of the mixer output from a standard frequency and to develop a control signal proportional to such deviation, which is supplied to the local oscillator to modify the frequency of the signal supplied by it to the mixer.

According to the invention, the discriminator output is gated, or alternatively the discriminator is rendered operative intermittently, to provide control signals which are determined by the mixer output only during timespaced intervals of relatively short duration. in a radar system, for example, these may correspond to the intervals during which pulses of high frequency energy are radiated by the transmitter. The application of these signals to the local oscillator is delayed in time by an amount of the order of, and preferably at least as great as, the durations of the intervals during which they are developed. It is to be noted that this delay may be ineluded anywhere in the loop circuit comprising the mixer, the discriminator, the local oscillator and their interconnections, each with the other. Thus, if desired, a substantial portion of the delay may be included in the connection from the mixer to the discriminator, lor it may be found that the'inherent delay around the loop will suce.

The relation between the delay in the application of the control signal and the duration of the intervals :in which control signal is developed is of the essence of the invention. By appropriate,determination of this relationship it is possible to prevent a change in the local oscillator frequency in response to a change in the output frequency from the mixer until after the interval during which the discriminator is operative to develop a control signal. This means that the ultimate shift in localroscillator frequency will, if the circuits are properly adjusted,

be exactly that required to compensate for the deviation in frequency of the mixer output from standard. lt will not be reduced, as in an ordinary AFC'system, by the gradual reduction in magnitude of the control signal as the oscillator frequency approaches the value required for exact compensation. In other words, the objective will be achieved that the relation between output and input of the mixer, as determined by the frequency of the local oscillator at any particular time, will depend almost solely on the output of the mixer at some preceding time and will be substantially independent of the change in said relationship between said times.

A complete understanding of the invention and of the organization and operation of a representative embodiment thereof will be gained by reading the following description with reference to the accompanying drawings `in which Figure 1 is a functional diagram of a so-called single-shot object motion indicating system embodying theinvention; Figure 2 is a schematic representation of circuits in accordance with the inventioruwhich Vmay be.

used in the system according to Figure l; Figure 3 is a.

schematic diagram illustrating the details of a variable gain D.C. amplifier suitable for use in the circuits of Figure 2; and Figure 4 illustrates schematically a modication of a portion of the circuits according to Figure 2.

Referring now to Figure l, in the single-shot radar system there functionally represented, P. R. F. (pulse repetition frequency) oscillator 1 is adapted to generate time-spaced electrical pulse signals having durations of 7 and 20 microseconds respectively, pulses of both durations being recurrent at a repetition frequency suitable for radar purposes-e. g. one kilocycle per second.l `These pulses may be of substantially rectangular form as indicated at a and b, and their leading edges, as shownmay be substantially coincident in time.

T he shorter pulses are supplied through connection 2 to magnetron oscillator 3 to control the generation by it of corresponding time-spaced pulses of energy which may be of a frequency in the microwave range-e. g. 30,000 megacycles per second. Thesepulses in turn are supplied through connection 4 to the input of a band-pass lter 5, which, for purposes of the single-shot operation here under consideration, may have a characteristic of transmissivity versus frequency as represented at c, the band- Width between half power points being, for example, 0.08 mega-.cycle The output of the band-pass filter will comprise pulses of rounded waveform at the pulse repetition frequency, consisting of frequency components primarily within a relatively narrow band e. g. 0.075 megacycle) as determined by the width of the filter pass band and the spectral distribution of the energy supplied by magnetron oscillator 3. Each of these pulses will be approximately 10 microseconds in duration between half-power points.

These pulses are supplied through connections 6 and 6a to duplexer 9an automatic high frequency switching device which may be constructed in accordance with usual radar practice. It is adapted, during the occurrence of each pulse of high frequency energy supplied from the output of band-pass filter 5, to transmit such pulses to the common'trausmitting and receiving antenna Y 10 to be radiated. At the same time it is adapted` to prevent high-level, high-frequency energy in any appreciable amount from entering the circuits of mixer A. The latter is Vadapted to be supplied through connection 1l with reflections of transmitted energy from target objects intercepted by antenna 10. Accordingly, duplexer 9 is also adapted, during the major portions of intervals between transmitted pulses, to permit the transmission of energy intercepted by antenna 10 through connection 1l to mixer VA. To these ends it may be constructed in accordance with conventional practice as fully set forth in numerous texts and technical articles, and may employ one or more T-R tubes, such as are well known in the art. (A T-R tube comprises essentially a pair of electrodes separated by a discharge gap and enclosed in a glass envelope containing air and water vapor at a pressure of the order of one millimeter of mercury.) Y

By means of an attenuative coupling 7 included in connections 6 and 6a from band-pass filter 5 to duplexer 9, a small amount of energy from each transmitted pulse is abstracted and supplied through connection 8 to the input of mixer B. Although this coupling is included in the connection 6, 6a from filter 5 to duplexer 9, it is not intended to attenuate the energy flowing in this path. ln the event that connections 6, 6a and 8 are wave guildes (as is customary at these frequencies), the attenuative coupling may consist merely of a relatively small hole in Athe waveguide comprising connections 6 and 6a at the point of junction between it and the waveguide comprising connection 8. Mixer B may be of the usual type, employing, for example, a silicon crystal. To its input there is also supplied, through connection 13a from local oscillator 13, a signal whose nominal 1 through Yconnections 16 and 17.' So long as the freqericy of the heterodyned received signals from the outp'utlof mixer A corresponds' to the nominal tl-megacycle I.-.-F..center frequency, no resultant output will be produced by' subtractor 26 during the intervals between successive transmitted pulses. However, if the signals intercepted by antenna 1li are caused to differ in either sense from the transmitted frequency owing to reection from moving target objects, the heterodyne output from mixer A will deviate correspondingly from the nominal l.-F. center frequency and l.-F. amplifiers A and B will operate on these signals in the same manner as in the case of deviations in the yfrequency or" the output from mixer B during the intervals corresponding to transmitter pulses.

., .Thus the output from subtractor 26, during intervals between transmitted pulses and in the presence of reections from moving target objects, will be indicative of the presence of such moving objects,A and the direction of their-motion will be indicated by the polarity of the output from-the subtractor. These signals are supplied through connection 27 to any suitable indicator circuits 28 adapted to present this information in a manner which may be in accord with standard practice for systems of this sort.V lt is to be noted that, owing to the fact that clamping circuit 32 is rendered responsive only during the occurrence of transmitted pulses to resultant signals appearing in the output of subtractor 26, the control signal developed thereby and applied to local oscillator 13 will not be altered during the intervals between successive transmitted pulses by resultant signals present in the output of subtractor 26 owing to signals received from moving objects. Rather the local oscillator frequency will tend to remain constant during each interval between transmitted pulses and during the transmitted pulse which immediately follows, and will be altered to conform to the new transmitted frequency immediately following transmission. Brief mention may be made of the reasons for employ ing separate mixers A and B to achieve mixing of the local oscillator signal with the transmitted and received signals respectively. Although it might be possible to use only a single mixer (mixer A) and to rely on the inherent leakage through duplexer 9 to supply signal from magnetron oscillator 3 to the input of mixer A, this arrangement appears to 'oe less desirable than the one described. This is owing to the fact that, during the intervals ot transmission by the system, duplexer 9 will be a source of spurious signals generated in the cavities of the T-R tubes. These spurious signals would tend to affect adversely the operation of the AFC circuits.

ln the arrangement shown in Figure 2 there is provided a waveguide structure comprising the two sections 50 and 5i, ends of which are joined to an end of a third ,section 52 by means of a conventional Y-junction 53. Section Sti may be supplied at its upper end with received signalsfor example from duplexer 9 of the system according to Figure l; while section 51 may bc supplied at its upper end with a portion of the transmitter energyfor example from the attenuative coupling 7 of the arrangement according to Figure l. The lower end of section 52, as illustrated, may be connected to the cavity resonator 56 of a velocity-modulation type local oscillator tube 57. Disposed within waveguide sections 50 and 51 at points interjacent their extremities are silicon crystals 54 and 55 respectively. Crystal 54 is adapted to mix the incoming signal supplied to the upper end of waveguide section 5t) with local oscillator signal supplied via section 52, `l-junction 53 and the lower portion of waveguide section S9. To prevent any appreciable amount of input power from the duplexer from proceeding beyond the point at which crystal 54 is located and being dissipated in the local oscillator cavity, an iris 50a -is inserted in waveguide Si) at a point approximately onequarter wavelength Afrom crystal 54 in the direction of -the oscillator cavity. Also, to regulate the amountof local oscillator power supplied to crystal 5 4, there is provideda controllable attenuator comprising a strip 50b of lossy` material A(e. g. Bakelite) which enters waveguide section- 50 through a slot in its wall 'interjacent crystal 54 and local oscillator cavity 56. The resultant heterodyne signal produced byV crystal mixer 54 is supplied through a transmission line section 58 to the primary winding of transformer 59. p v v YLikewise, crystal 55 is adapted to mix the portion of the transmitter energy supplied to the upper end Vof waveguide scctionl with local oscillator signal supplied through waveguide section 52, Y-junction 53 and the lower part of waveguide section 51. To regulate the amounts of transmitter andlocal oscillator energy supplied to crystal 55, controllable attenuators are provided in waveguide section 51 on both sides of crystal 55.

These take the form of strips 51a and 51b of lossy material inserted into the waveguide through slots in its wall.

The heterodyne signal produced in crystal mixer 55 isV transmission of the two heterodyne signals produced inV crystal mixers 54 and 55 to the tuned primary winding 70 of a transformer having separate tuned secondary windings 7 and 72. To control such alternate transmission, tube 61 is normally biased to conduct, While tube 62 is normally biased to a condition of cut-off. Negative pulses, coinciding in time with the intervals of transmission of the radar system, are supplied through cou` pling condenser 63 to the screen grid of tube 61 and through isolating resistor 64 and coupling condenser 65 to the cathode of tube 62. During the occurrence of such negative pulses, tube 61 will be caused to cut olf and tube 62 will be rendered conductive so as to transmit heterodyne signals from crystal mixer 55 to the transformer primary Winding 70.

Regardless of the condition of tubes 61 and 62, the signals appearing in the primary winding 70 will be transferred to both of the tuned secondary windings 71 and "/"2 which are connected respectively to the inputs of I.-F. ampliierA and if?. ampiier B. The gain versus frequency characteristics of the-se amplifiers are as illustrated at d and e respectively and correspond to those already discussed with reference to the embodiment of the invention shown in Figure l. The method of designing ampliers capable of providing such characteristics is well understood, and hence it is not considered necessary to illustrate the amplifier circuits in detail. The outputs of the two intermediate frequency amplifiers are connected respectively to the primary windings of transformers 73 and 74. To the secondary windings 7S and 76 of these transformers are connected conventional detector circuits' which are here shown as comprising germanium crystals 77 and 8), condensers '79 and 82 and load resistors 78 and 8l. A connection is provided between the lower terminal of resistor 78 and the upper terminal of resistor 8l, and crystals 77 and Si) are connected in their respective circuits in opposite senses so that resultant signal, developed across resistors 78 and S1 in series, Vwill be equal to the difference between the separate 1detected signals appearing across resistors 78 and 8i individually. These resultant signals may be supplied through connection 84 to suitable indicator circuits as in the arrangement according to Figure l. Also, as in the arrangement of Figure l, the signals developed across resistors 7S and 81 in series, during the intervals in whichY gating tube 62 is conducting, may be delayed and used to control the frequency of signal generated by the localoscillator tube 57. f

* The localoscillatorY frequency is .conveniently controlied by varying the potential of the repeller electrode of tube 57. To this end there, is provided a condenser 85 in series with the conneciion from the repeller electrode to a tap on the D.C. power supply 1th?. During the intervals corresponding to transmission of pulses of high frequency energy by the radar system, the charge on condenser 85 is increased or decreased by an amount which may be made substantially proportional to the resultant signal developed across resistors 78 and 81 in series. This is accomplishesl through the agency of triode vacuum tubes 86, 37 and 3 which are connected in series, the cathode of tube S6 being connected to the plate of tube 87 and the cathode of tube 37 being connected to the plate of tube S3. D.C. power for the operation of these tubes is supplied through connections 99 and 99a from the plate of tube 5 and the cathode of tube SS respectively to taps on D.C. power supply 10G. rFue latter also selves as power supply for local oscillator tube 57. As illustrated, one plate of con` denser 8S is connected through connect-ion 101 to the junction between the cathode of tube 87 and the plate of tube S8.

Tubes 37 and 88 are rendered conductive during intervals delayed with reference to those of transmission by the radar. To this end, the 20-microsecond pulses from P. R. F. oscillator 1 of Figure l are delayed in a delay line S9 comprising condensers 90 and inductors 91. The output of the delay line is supplied through condenser 92 to the grid of a driver tube 93, in the output circuit of which is connected the primary winding of a transformer 94. From the secondary windings of transformer 94: positive 20microsecond pulses are supplied to the grids of tubes 87 and SS. During the occurrence of these positive pulses, tubes 87 and Sii are rendered conductive; while during the intervals between such pulses, tubes 87' and 38 are cut off owing to the charge accumulated on grid condensers 95 and 96 during the intervals of conduction.

While tubes 87 and S8 are conducting, the current through them is determined in accordance with the potential applied to the grid of tube 36. in order that this current may be substantially proportional to the resultant signal appearing across resistors 7d and 31, the latter signal is supplied through delay line 103 and variable gain D.C. amplifier 106 to the input of tube 86. Delay line 103 comprises condensers 102 and inductors 104 and is terminated in an impedance 16S of appropriate value. The delay introduced by it should be such that the over-all delay around the AFC loop is at least as great as the durations of the intervals during which tubes 87 and S3 are conductive. However, if there is no resultant signal across resistors 78 and 81 (as will be the case when local oscillator 57 is operating at a frequency such as to compensate exactly for any deviation in the transmitter frequency), it is desired that condenser 85 be neither charged nor discharged during the conductive intervals of tubes 36 and S7. Accordingly the potential developed across condenser is supplied through a variable gain D.C. ampliiier 107 and through a delay line 108- in series with the input to tube S6. Thus the current in tubes S7 and 88, durin their conductive intervals, is controlled in response to the potential across condenser 85. By appropriate adjustment of variable gain D.C. amplifier 107, it may be controlled in such a manner that no charging or discharging of condenser S5' will take place except when additional input to tube 36 is supplied rom the output of variable gain D.-C. ampliiier 106. By the inclusion of delay line 103, changes in the potential across condenser' d5 in response to this input are prevented from being neutralized. To this end the delay in application of the potential from condenser 85 to the input of tube 86 should be at least as great as the duration of the intervals during which tubes S7 and 8S are conductive.

The variable gain D.-C. ampliers 106 and 107 maybe constructed as shown in Figurev 3. Here the input signal to be amplified, and whose D..C. component is to be preserved, is applied to the control grid of a pentode vacuum tube 130, and the output signal appearing across load resistor 109 is modulated upon a carrier of suitably high frequency (e. g. 20 megacycles) supplied from a source 111 through a transformer 112. Modulation is effected by means of a germanium crystal 110, and the resultant signal is supplied through a transformer 113 to the input of a conventional variable gain ampliiier 114. The output from this amplifier appearing across the secondary winding of transformer 115 is detected by means of a circuit comprising a second germanium crystal 116, a condenser 117 and a load resistor 118. The amplified output appearing across resistor 118 includes the D.-C. component contained in the input signal.

By reasonably careful adjustment of the gain of amplifiers 106 and 107 in the arrangement according to Figure 2, it is possible to cause the AFC system to correct almost instantaneously and exactly for departures in the frequency of signals supplied to the input end of waveguide 54. This adjustment is readily made by varying the gains of amplifiers 106 and 107 until the mean frequencies of heterodyned received signals from iixed targets supplied to the inputs of I.F. amplifiers A and B correspond at all times with the null points of their respective characteristics as represented at d and e. Even if the adjustment is not exactly made for this optimum control, correction will, in general, be effected more rapidly and more precisely `than in known systems, provided only that, in any given instance, the increment or decrement of charge on condenser does not exceed twice the amount required exactly to correct the frequency of local oscillator 57.

It may be mentioned that, when the adjustments re ferred to in the previous paragraph are made upon ampliers of the form illustrated in Figure 3, it may be necessary to readjust the potentials supplied to the elements of tube 57-particularly the cathode potential. The need for such adjustment could be obviated by using a balanced version of the amplifier shown in Figure 3.

If desired some simplification can be made in the circuits of the arrangement according to Figure 2, while still retaining in large part the advantages of the more complicated circuit. For example, for the portion of the circuit of Figure 2 enclosed within broken line N. there may be substituted thesimpler arrangement shown in Figure 4. In making this substitution the terminals H, K, L and M in Figure 4 may be connected at the simi larly designated points in Figure 2. In Figure 4, triodes 123, 124 and 125 correspond respectively to triodes 86, S7 and S3 of the arrangement according to Figure 2 and perform substantially the same functions. Likewise, local oscillator tube 119 of Figure 4 corresponds to local oscillator tube 57 of Figure 2 and is similarly connected. However, in the arrangement according 'to Figure 4, variable gain D.C. amplifier 107 and delay line 108 of Figure 2 are omitted. Thus no means is provided for controlling the current in tubes 124 and 125 during their conductive intervals in accordance with the potential developed across condenser 121. Hence, regardless of the frequency at which local oscillator 119 is operating, as determined by the charge on condenser 121, if the resultant output from resistors 7S and S1 of the arrangement according to Figure 2 is zero and no signal is applied to the grid of tube 123, the current through tubes 124 and 125 will be the same and condenser 121 may be either charged or discharged. To minimize this effect, resistor 122 is included in the connection from condenser 121 to the junction between the cathode of tube 12d and the plate of tube 125. The time constant of the RC circuit comprising resistor 122 and condenser 121 is made large compared to (e. g. times) the duration of the intervals during which tubes 124 and 125 are conductive. By this means the effect on the charge on condenser 121 during intervals in whichno signal'is applied to the grid of tube 123 is minimized. At the same time variations in the current through tubes 124 and v125 will be effective to alter the charge on condenser 121 so as to control the frequency of local oscillator 119.

It is to be noted that, in the system as described with reference to Figures l and 2, LF. amplifiers A and B and their associated circuits serve both to detect departures in frequency of received signals indicative of moving objects, and also, during the transmitted pulse intervals, as an AFC discriminator. By utilizing the same circuits in this manner to perform both functions, there is avoided the necessity for providing two circuits which would have to be carefully tuned to the same frequency. This feature is a direct result of the mode of operation in accordance with the invention, whereby the AFC circuit operates only during intervals in which received signals are not required to be detected.

As has been emphasized, the various time relationships involved in circuits according to the invention are of considcrable significance. Accordingly it is appropriate to refer to the values of components to be employed in the circuits according to Figures 2, 3 and 4 with the caution, however, that these are not to be regarded as necessarily applicable except under the specific conditions hereinbefore recited, namely:

Transmitter frequency mc 30,000 Pulse repetition rate cycles.- 1,000 Transmitted pulse length (between half-power points) nsec..- Transmitted bandwidth (between half-power points) me 0.075 Intermediate frequency mc 60 For these conditions, the component values may be as follows:

Tubes and crystals as designated in Figure 2.

It is, of course, to be understood that, although the invention has been described with reference to but a single representative embodiment, which adequately illustrates the principles involved and the method of applying them, these principles are susceptible of application in numerous other instances, and employing other arrangements which, nevertheless, lie Within the scope of the invention as defined by the appended claims. v

We claim:

l. in an automatic control system, a source of original signals controllable in respect of a predetermined parameter thereof, discriminatory means responsive to said original signals for producing control signals having a magnitude and polarity indicative of the magnitude and sense of departures of said predetermined parameter of said original signal from a predetermined value of said parameter, control means responsive to said control signals for controlling said source to vary said predetermined parameter of said original signals in a sense to reduce said departures and hence to reduce said control signals, said control means being normally inoperative, means independent of variations in said predetermined parameter for rendering said control means intermittently operative to control said original signals during timespaced intervals of durations not exceeding a predetermined maximum, and means for delaying the substantial etfect'of said control upon said control signal for a time at least equal to said predetermined maximum duration.

2. The system of claim l, in which said control means includes a circuit arrangement for maintaining said predetermined parameter of said original signals at a substantially constant value in the periods between said timespaced intervals.

3. The system of claim 1, in which said means for rendering said control means operative comprises a source of periodically-recurrent signals and means for supplying said last-named signals to said control means to render said control means operative during time-spaced intervals which are periodically recurrent.

4. The system of claim 1, in which said source of original signals comprises means responsive to an input signal for producing an output signal differing from said input signals in respect of said predetermined parameter, but possessing variations in respect of said parameter which are controllably related to those of said input signals.

5. In an automatic frequency control system, a `source Y of original signals of controllable frequency, a frequency discriminator responsive to said original signals for producing control signals having a magnitude and polarity indicative of the magnitude and sense of differences between said frequency of said original signals and a predetermined reference frequency, frequency control means responsive to said control signals to vary said frequency of said original signals in a direction to reduce said difference from said reference frequency and hence to reduce said control signals, said last-named means being normally inoperative, means for rendering said last-named means intermittently operative during time-spaced intervals not exceeding a predetermined maximum, and means for delaying the substantial effects of said variation of said frequency of said original signals upon said control signals for a time at least equal to said predetermined maximum duration.

6. The system of claim 5, in which said source of original signals comprises means responsive to a first input signal and to a second locally-generated signal for producing an output signal whose frequency differs from that of said first signal by an amount determined by the frequency of said second signal, and in which said frequency control means is operative to control the fre quency of said second locally-generated signal.

7. The system of claim 5, comprising in addition means for maintaining said control signal at a substantially constant value between said time-spaced intervals of predetermined duration.

8. The system of claim 5, in which said means for rendering said control means operative comprises a source of periodically-recurrent signals for rendering said control means intermittently operative during periodicallyrecurrent time-spaced intervals.

9. In an automatic frequency-control system, a source of original signals of controllable frequency, a frequency discriminator responsive to said original signals to produce output signals having a magnitude and polarity indicative of the magnitude and sense of departures of said frequency of said original signals from a predetermined reference frequency, control means responsive to input signals applied thereto for supplying control signals to said source of original signals to vary the frequency of said original signals in such sense as to reduce said frequency departures, controllably-actuatable electronic switching means for intermittently supplying said discriminator output signals to said control means during time-spaced intervals substantially contemporaneous with the occurrences of actuating signals applied thereto, means `for generating actuating signals comprising a series of separate time-spaced pulses of durations not exceeding a predetermined maximum, means for supplying said actuating signals to said switching means to effect application of said discriminator output signals to said control means during the occurrences of said separate pulses, and means for delaying the substantial effects of modifications of said control signals upon said discriminator output signals for a time at least equal to said predetermined maximum duration.

10. In an automatic frequency control system, a mixer responsive to an input signal and a locally generated signal to produce an output signal whose frequency is related to the frequencies of said input and locally generated signals, a local oscillator arranged to supply said locally generated signal to said mixer, a discriminator arranged to be supplied with an output signal from said mixer and operative to develop a control signal indicative of variations in the frequency of said output signal, means for supplying said control signal from said discriminator to said local oscillator to control the frequency of oscillation of said local oscillator, said last-named means being normally inoperative, means for rendering said last-named means operative intermittently to supply said control signal from said discriminator to said local oscillator during timespaced intervals of durations not exceeding a predetermined maximum, and means for delaying the supply of said control signal from said discriminator to said local oscillator by an amount such as to prevent any substantial change in frequency of the output signal from said mixer in response to the control of said local oscillator during 14 said intervals when said control signal supply means is operative.

11. In an automatic control system, a pair of vacuum tubes having their discharge paths connected in series, an energy storage device connected electively in shunt with one of said tubes, whereby the quantity of energy stored therein is determined in accordance with current through said tubes when they conduct, means for rendering said tubes conductive or non-conductive simultaneously, means for establishing a controllable current in said tubes when they are conductive, means responsive to a control signal for controlling said current, and means including a signal delay device responsive to the quantity of energy stored in said energy storage device for further controlling said current.

References Cited in the tile of this patent UNITED STATES PATENTS 2,333,688 Shepard Nov. 9, 1943 2,400,309 Kock May 14, 1946 2,422,082 Crosby June 10, 1947 2,425,013 Stotz Aug. 5, 1947 2,428,799 Hayes Oct. 14, 1947 2,433,350 Earp Dec. 30, 1947 2,434,293 Stearns Jan. 13, 1948 2,434,294 Ginzton Jan. 13, 1948 

